1. Field of the Invention
Embodiments of the present invention relate to electrolytic chemical mechanical polishing. More particularly, embodiments of the present invention relate to an electrolyte solution and apparatus for copper removal and a method for removing copper ions therefrom.
2. Background of the Related Art
Reliably producing sub-half micron and smaller features is one of the key technologies for the next generation of very large scale integration (VLSI) and ultra large-scale integration (ULSI) of semiconductor devices. However, as the limits of circuit technology are pushed, the shrinking dimensions of interconnects in VLSI and ULSI technology have placed additional demands on the processing capabilities. Reliable and repeatable formation of these interconnects is important to VLSI and ULSI success and to the continued effort to increase circuit density and improve process yield.
Multilevel interconnects are formed using sequential material deposition and material removal techniques on a substrate (or wafer) surface to form features therein. As layers of materials are sequentially deposited and removed, the uppermost surface of the substrate may become non-planar across its surface and require planarization prior to further processing. Planarization or “polishing” is a process where material is removed from the surface of the substrate to form a generally even, planar surface. Planarization is useful in removing excess deposited material and removing undesired surface topography and surface defects, such as rough surfaces, agglomerated materials, crystal lattice damage, scratches, and contaminated layers or materials to provide an even surface for subsequent lithography and processing.
Electrochemical mechanical polishing (ECMP) is one method of planarizing a surface of a substrate. ECMP removes conductive materials from a substrate surface by electrochemical dissolution while polishing the substrate with a reduced mechanical abrasion compared to conventional chemical mechanical planarization (CMP) processes. A typical ECMP system includes a substrate support, an anode (generally the substrate), and cathode disposed within an electrolyte containment basin. In operation, metal atoms on a surface of a substrate are ionized by an electrical current from a source of potential, such as a battery or other voltage source connected to the two electrodes. The metal ions dissolve into the surrounding electrolyte solution at a rate proportional to the electric current. The metal ions from the substrate (anode) either plate the electrode (cathode), fall out of the solution as a precipitate, or remain in the solution. The destiny of the metal ions depends greatly on the chemistry of the metals and the solution.
Due to the push for high tool throughput, processed wafers per hour, the. goal in ECMP type processes is to maximize the electrochemical dissolution rate of the desired material from the surface of the substrate. The dissolution current (i.e., dissolution rate) is limited by some fundamental aspects of the electrochemical process. It is important to understand, in electrochemical processes such as ECMP, that charge neutrality in the electrolyte must be conserved (in a non-transient process or process step) and thus for every metal ion removed from the anode (surface of the substrate), a corresponding number of electrons are accepted at the cathode by a positive ion(s). At low to moderate electrochemical currents, the process will typically cause the metal ions in the electrolyte solution to be plated onto the cathode surface. Although, if the rate at which the metal ions are removed from the anode surface is increased, the reaction at the cathode can become limited since the fluid near the surface of the cathode becomes depleted of the metal ions. The electrochemical reaction, therefore, becomes rate limited by the process of diffusion of the metal ions across the depleted region around the cathode. Since the current is limited by the diffusion process of the ions, this state is commonly known as the limiting current (iL). The depleted region near the surface of the cathode is commonly known as the diffusion boundary layer, or Nernst diffusion boundary layer, which is related to the hydrodynamic boundary layer. If the applied voltage, or driven current, is then further increased, a secondary reaction such as gas evolution, can occur at the cathode and/or anode, depending on the chemistry, applied voltage, hydrodynamics, etc., which then allows the electrical current to be further increased. As an example, in acidic chemistries, e.g., baths with a low pH, it is common for the hydrogen ions to be reduced at the cathode, thus causing hydrogen gas to be evolved. A discussion of electrochemical processes such as described here can be found in Chapters 1-11 of “Electrochemical Methods,” by Alan J. Bard published by John Wiley & Sons, 1980.
One problem associated with the gas evolution is that the generated gas forms bubbles that can become trapped against the surface of the anode, cathode, flow diffusers, membranes, and/or some intermediate surfaces in an electrochemical cell. The trapped gas, due to its insulating properties, tends to adversely affect the electrochemical process. In one case, the bubbles can cause an increase in the total cell electrical resistance, since the bubbles tend to decrease the area in which the current can pass through membranes, anode surface or the cathode surface, due to their insulating properties. In another case, where the anode surface is face down in the electrochemical bath, the evolved gas can tend to rest on the substrate or pad surfaces preventing dissolution from these covered areas. The covered surfaces can, thus, leave unpolished or non-uniform polished areas on the substrate, leaving defects that can affect subsequent semiconductor processes.
There is a need, therefore, for various apparatus designs and chemistries which will prevent or reduce the amount of gas generated during the ECMP process and reduce the detrimental effect of the gas bubbles on the ECMP process.